Thanks for the replies. I will reply in English so it's understandable for everyone.
First of all, I cannot use only / in this case, because the real address where the server is located is, estrellaserver.no-ip.info. It only looks like the address is server.estrellaweb.nl because the no-ip address opens through the frame. But the point is. If I will use / instead of the full address the page 'thinks' the images are located on the no-ip address, which isn't.
But, I checked for the quotes and all images are now surrounded by them.
About the doctype stuff. To be honest with you. I don't know a thing about that. Some say every HTML page should have a doctype but I'm now sure which one and how you can determine which doctype is OK for your page. The doctype currently in my page... I guess it already has been there. I didn't put it in myself, because of the simple reason I don't know anything about it.
I completely removed the Doctype line, added some quotes where needed but still the same prob. :-(